The present invention relates to acoustic detection apparatus and more particularly to apparatus for determining the direction of propagation of an acoustic signal emanating from a remote source.
Passive detection of distant man-made objects or acoustic sources, such as submarines, in a typically noisy medium is a well recognized problem in the art. Characteristically, man-made objects emanate acoustic signals which are generally coherent, periodic and localized as opposed to the aperiodic acoustic signals randomly appearing throughout a natural environment. Thus, in antisubmarine warfare in particular, it has been the general practice in the prior art to utilize the characteristic distinctions between the signal patterns of a man-made object, such as the patterns generated by a submarine propeller, and the random patterns distributed within the ocean. Such passive submarine detection systems operated on the principal distinctions between random and non-random processes, generally performed by correlation or integration of acoustic signals over predetermined intervals of time where the random signal characteristically exhibit substantially no correlation or substantially integrate to zero. The man-made signal, on the other hand, is generally correlated and integrates to a non-zero value. Accordingly, the various techniques devised in the past utilize this distinction to identify the presence of a man-made source, being combined with phase sensitive devices in order to determine the direction of the source relative the detection system. Since the presence of the man-made source in these systems is determined by inspecting acoustic signals coming from all directions the effective signal-to-noise ratio of the periodic man-made signal, which is typically localized, against the omnidirectional background noise is low and extensive processing is required in order to enhance the man-made signal. Thus, it has been the general practice in the prior art to statistically compare localized coherent signals against background noise received from all directions where typically the randomness of stationary background noise does not vary with azimuth.